System and method for controlling work machine

ABSTRACT

A controller determines a plurality of candidate paths. The plurality of candidate paths cross a first digging wall from a first slot toward a second slot and extend in respective directions. The controller calculates an evaluation function of a path search algorithm for each of the plurality of candidate paths. The controller determines a candidate path having an optimal evaluation function of the plurality of the candidate paths as a first digging path. The controller controls a work machine according to the first digging path.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National stage application of International Application No. PCT/JP2020/045518, filed on Dec. 7, 2020. This U.S. National stage application claims priority under 35 U.S.C. § 119(a) to Japanese Patent Application No. 2019-235832, filed in Japan on Dec. 26, 2019, the entire contents of which are hereby incorporated herein by reference.

BACKGROUND Technical Field

The present disclosure relates to a system and a method for controlling a work machine.

Background Information

Slot dozing is work performed by a work machine. In slot dozing, an actual topography of a work site is dug by a work implement, whereby a plurality of slots are formed on the actual topography. Moreover, digging walls are formed between the plurality of slots. The digging walls are berms left along the slots.

U.S. Pat. No. 9,469,967 describes a start condition for work to dig and remove the digging walls. For example, the controller determines whether to start digging of a digging wall based on the difference in the depths of the slots adjacent to the digging wall on both sides or the width of the digging wall.

SUMMARY

However, a specific operation of the work machine for digging the digging walls is not disclosed in U.S. Pat. No. 9,469,967. The digging work of the digging walls requires skill. Therefore, the digging work of the digging walls is not easy for an inexperienced operator. An object of the present disclosure is to easily perform digging work of digging walls with automatic control of the work machine.

A system according to a first aspect of the present disclosure is a system for controlling a work machine. The system according to the present aspect includes a position sensor and a controller. The position sensor outputs position data indicative of a position of the work machine. The controller acquires the position data. The controller acquires actual topography data. The actual topography data includes a position of a first slot, a position of a second slot, and a position of a first digging wall. The first slot extends in a predetermined work direction. The second slot is positioned adjacent to the first slot. The first digging wall is positioned between the first slot and the second slot.

The controller determines a plurality of candidate paths. The plurality of candidate paths cross the first digging wall from the first slot toward the second slot and extend in respective directions. The controller calculates an evaluation function of a path search algorithm for each of the plurality of candidate paths. The controller determines a candidate path having an optimal evaluation function of the plurality of candidate paths as a first digging path. The controller controls the work machine according to the first digging path.

A method according to a second aspect of the present disclosure is a method for controlling a work machine. The method according to the present aspect includes the following processes. A first process is to acquire position data indicative of a position of the work machine. A second process is to acquire actual topography data. The actual topography data includes a position of a first slot, a position of a second slot, and a position of a first digging wall. The first slot extends in a predetermined work direction. The second slot is positioned adjacent to the first slot. The first digging wall is positioned between the first slot and the second slot.

A third process is to determine a plurality of candidate paths. The plurality of candidate paths cross the first digging wall from the first slot toward the second slot and extend in respective directions. A fourth process is to calculate an evaluation function of a path search algorithm for each of the plurality of candidate paths. A fifth process is to determine a candidate path having an optimal evaluation function of the plurality of candidate paths as a first digging path. A sixth process is to control the work machine according to the first digging path. The order in which the above processes are executed is not limited to the order described above and may be changed.

According to the present disclosure, digging work of digging walls can be easily performed with automatic control of the work machine.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view of a work machine according to an embodiment.

FIG. 2 is a block diagram illustrating a configuration of a drive system and a control system of the work machine.

FIG. 3 is a side view of an actual topography at a work site.

FIG. 4 is a perspective view illustrating an example of slots and digging walls formed on the actual topography.

FIG. 5 is a flowchart illustrating processes of automatic control of the work machine.

FIG. 6 is a top view illustrating an example of the actual topography.

FIG. 7 is a view illustrating an example of a travel path including first to third travel paths.

FIG. 8 is a view illustrating the example of the travel path including fourth to sixth travel paths.

FIG. 9 is a view illustrating the example of the travel path including seventh to ninth travel paths.

FIG. 10 is an enlarged view of a first travel path and a second travel path.

FIG. 11 is an enlarged view of an m-th travel path.

FIG. 12 is a flowchart illustrating processes for determining a digging path.

FIG. 13 is a view illustrating an example of candidate paths of a digging path.

FIG. 14 is a block diagram illustrating a configuration of the drive system and the control system of the work machine according to another embodiment.

FIG. 15 is a view illustrating an example of the travel path including a first travel path according to a modified example.

FIG. 16 is a view illustrating the example of the travel path including a second travel path according to the modified example.

FIG. 17 is a view illustrating the example of the travel path including a k-th travel path according to the modified example.

DESCRIPTION OF EMBODIMENTS

A system and a method for controlling a work machine 1 according to an embodiment will be described with reference to the drawings. FIG. 1 is a side view of the work machine 1 according to the embodiment. The work machine 1 according to the present embodiment is a bulldozer. The work machine 1 includes a vehicle body 11, a travel device 12, and a work implement 13.

The vehicle body 11 includes an operating cabin 14 and an engine compartment 15. A operator's seat that is not illustrated is disposed in the operating cabin 14. The engine compartment 15 is disposed in front of the operating cabin 14. The travel device 12 is attached to a bottom part of the vehicle body 11. The travel device 12 has a pair of left and right crawler belts 16. Only the left crawler belt 16 is illustrated in FIG. 1 . The work machine 1 travels due to the rotation of the crawler belts 16.

The work implement 13 is attached to the vehicle body 11. The work implement 13 includes a lift frame 17, a blade 18, and a lift cylinder 19. The lift frame 17 is attached to the vehicle body 11 such as to be movable up and down. The lift frame 17 supports the blade 18.

The blade 18 is disposed in front of the vehicle body 11. The blade 18 moves up and down accompanying the up and down movements of the lift frame 17. The lift cylinder 19 is coupled to the vehicle body 11 and the lift frame 17. Due to the extension and contraction of the lift cylinder 19, the lift frame 17 moves up and down.

FIG. 2 is a block diagram illustrating a configuration of a drive system 2 and a control system 3 of the work machine 1. As illustrated in FIG. 2 , the drive system 2 includes an engine 22, a hydraulic pump 23, and a power transmission device 24.

The hydraulic pump 23 is driven by the engine 22 to discharge hydraulic fluid. The hydraulic fluid discharged from the hydraulic pump 23 is supplied to a hydraulic actuator 25. The hydraulic actuator 25 includes the above-mentioned lift cylinder 19. Although one hydraulic pump 23 is illustrated in FIG. 2 , a plurality of hydraulic pumps may be provided.

A control valve 26 is disposed between the hydraulic actuator 25 and the hydraulic pump 23. The control valve 26 is a proportional control valve and controls the flow rate of the hydraulic fluid supplied from the hydraulic pump 23 to the lift cylinder 19. The control valve 26 may be a pressure proportional control valve. Alternatively, the control valve 26 may be an electromagnetic proportional control valve.

The power transmission device 24 transmits driving force of the engine 22 to the travel device 12. The power transmission device 24 may be, for example, a transmission having a torque converter or a plurality of transmission gears. Alternatively, the power transmission device 24 may be another type of power transmission device, such as a hydro static transmission (HST).

The control system 3 includes a controller 31, a machine position sensor 32, a communication device 33, a storage 34, and an input device 35. The controller 31 is programmed to control the work machine 1 based on acquired data. The controller 31 includes a memory 38 and a processor 39. The memory 38 includes, for example, a random access memory (RAM) and a read only memory (ROM). The storage 34 includes, for example, a semiconductor memory, a hard disk, or the like. The memory 38 and the storage 34 record computer commands and data for controlling the work machine 1.

The processor 39 is, for example, a CPU, but may be another type of processor 39. The processor 39 executes processes for controlling the work machine 1 based on the computer commands and data stored in the memory 38 or the storage 34. The communication device 33 is, for example, a module for wireless communication and communicates with a device outside of the work machine 1. The communication device 33 may be a device that uses a mobile communication network. Alternatively, the communication device 33 may be a device that uses a local area network (LAN) or another network, such as the Internet.

The machine position sensor 32 detects a position of the work machine 1. The machine position sensor 32 includes, for example, a global navigation satellite system (GNSS) receiver such as a global positioning system (GPS). The machine position sensor 32 is mounted on the vehicle body 11. Alternatively, the machine position sensor 32 may be mounted on another position such as on the work implement 13. The controller 31 acquires current position data indicative of a current position of the work machine 1 from the machine position sensor 32.

The input device 35 is operable by an operator. The input device 35 includes, for example, a touch screen. Alternatively, the input device 35 may include another operating element such as hardware keys. The input device 35 receives an operation by an operator and outputs a signal indicative of the operation by the operator to the controller 31.

The controller 31 outputs command signals to the engine 22, the hydraulic pump 23, the power transmission device 24, and the control valve 26, thereby controlling said devices. For example, the controller 31 controls the displacement of the hydraulic pump 23 and the opening degree of the control valve 26 to operate the hydraulic actuator 25. As a result, the work implement 13 can be operated.

The controller 31 controls the rotation speed of the engine 22 and the power transmission device 24 to cause the work machine 1 to travel. For example, when the power transmission device 24 is an HST, the controller 31 controls the displacement of the hydraulic pump and the displacement of a hydraulic motor of the HST. When the power transmission device 24 is a transmission having a plurality of transmission gears, the controller 31 controls an actuator for gear shifting. Further, the controller 31 controls the power transmission device 24 so as to bring about a speed difference between the left and right crawler belts 16, thereby causing the work machine 1 to turn.

Next, automatic control of the work machine 1 executed by the controller 31 will be described. The controller 31 controls the engine 22 and the power transmission device 24, thereby causing the work machine 1 to travel automatically. Further, the controller 31 controls the engine 22, the hydraulic pump 23, and the control valve 26, thereby automatically controlling the work implement 13.

FIG. 3 is a side view of an actual topography 40 at a work site. As illustrated in FIG. 3 , the work machine 1 determines a target design surface 41. At least a portion of the target design surface 41 is positioned below the actual topography 40. The target design surface 41 extends in a predetermined work direction A1. The target design surface 41 may be determined in advance and stored in the storage 34. The controller 31 may determine the target design surface 41 from the actual topography 40. Alternatively, the target design surface 41 may be input by an operator via the input device 35.

The controller 31 determines a start position 101 of digging on the actual topography 40. For example, the controller 31 may determine the start position 101 based on the amount of soil to be dug. The controller 31 controls the work machine 1 to cause the work machine 1 to move from the start position 101 to a dumping position D1. As a result, the actual topography 40 is dug from the start position 101 and the dug soil is transported to the dumping position D1.

Next, the controller 31 causes the work machine 1 to move to a next start position 102 positioned behind the previous start position 101. Then, the controller 31 controls the work machine 1 to cause the work machine 1 to move from the start position 102 to the dumping position D1. As a result, the actual topography 40 is dug from the start position 102 and the dug soil is transported to the dumping position D1. By repeating the above operations, as illustrated in FIG. 4 , a first slot S1 extending in the predetermined work direction A1 is formed on the actual topography 40. The control for generating the first slot S1 is not limited to the control described above and may be changed.

The controller 31 controls the work machine 1, whereby a plurality of slots S1 to S4 are formed on the actual topography 40 in order. The plurality of slots S1 to S4 are aligned in a lateral direction. The lateral direction is a direction intersecting the predetermined work direction A1. The plurality of slots S1 to S4 are disposed apart from each other. Therefore, digging walls W1 to W3 are formed between the slots S1 to S4. The following is an explanation of automatic control of digging work of the digging walls W1 to W3 performed by the work machine 1 at the work site.

FIG. 5 is a flowchart illustrating processes of automatic control of the work machine 1. As illustrated in FIG. 5 , in step S101, the controller 31 acquires current position data. The controller 31 acquires the current position data from the machine position sensor 32.

In step S102, the controller 31 acquires actual topography data. The actual topography data is data indicative of the actual topography 40 at the work site. For example, the actual topography data includes plane coordinates and heights of a surface of the actual topography 40. The actual topography data includes positions of the above-mentioned slots S1 to S4 and digging walls W1 to W3.

FIG. 6 is a top view illustrating an example of the actual topography 40. The controller 31 performs digging work of the digging walls W1 to W3 in a work area 100 at the work site. The work area 100 may be determined in advance and stored in the storage 34. The work area 100 may be automatically determined by the controller 31. Alternatively, the work area 100 may be input by an operator via the input device 35.

In the example illustrated in FIG. 6 , the actual topography 40 in the work area 100 includes the first to fourth slots S1 to S4. The first to fourth slots S1 to S4 extend in the predetermined work direction A1. The first to fourth slots S1 to S4 are aligned in the lateral direction. The first to fourth slots S1 to S4 are disposed apart from each other. The actual topography 40 in the work area 100 includes the first to third digging walls W1 to W3. The first digging wall W1 is positioned between the first slot S1 and the second slot S2. The second digging wall W2 is positioned between the second slot S2 and the third slot S3. The third digging wall W3 is positioned between the third slot S3 and the fourth slot S4. The first to third digging walls W1 to W3 extend in the predetermined work direction A1.

The actual topography data may be stored in the storage 34 in advance. The controller 31 may acquire the actual topography data by recording trajectories of the work implement 13 or the bottom of the travel device 12. Alternatively, the actual topography data may be measured by a measuring device such as a laser imaging detection and ranging (LIDAR) or a camera. The controller 31 may acquire the actual topography data from the measuring device. The measuring device may be mounted on the work machine 1. The measuring device may be disposed outside of the work machine 1.

In step S103, the controller 31 acquires dumping positions D1 to D4. The dumping positions D1 to D4 are positioned in front of the slots S1 to S4 in the predetermined work direction A1. In the example illustrated in FIG. 6 , the dumping positions D1 to D4 include the first to fourth dumping positions D1 to D4. The first to fourth dumping positions D1 to D4 are respectively positioned in front of the first to fourth slots S1 to S4 in the predetermined work direction A1.

In step S104, the controller 31 determines travel paths P1 to Pn. FIGS. 7 to 9 are views illustrating an example of the travel paths P1 to Pn. As illustrated in FIGS. 7 to 9 , the controller 31 determines a plurality of travel paths P1 to Pn for dumping all the digging walls W1 to W3 in the work area 100 at the dumping positions D1 to D4. In the example illustrated in FIGS. 7 to 9 , the controller 31 allocates the first to n-th travel paths P1 to Pn to the digging walls W1 to W3 in order in the lateral direction.

Specifically, as illustrated in FIG. 7 , the controller 31 allocates the first to third travel paths P1 to P3 to the first to third digging walls W1 to W3 in order. As illustrated in FIG. 8 , the controller 31 reverses the order of allocation in the opposite direction at the third digging wall W3. The controller 31 allocates the fourth to sixth travel paths P4 to P6 in order from the remaining portion of the third digging wall W3 to the remaining portion of the first digging wall W1. As illustrated in FIG. 9 , the controller 31 reverses the order of allocation in the opposite direction at the first digging wall W1. In the same manner as described above, the controller 31 allocates the seventh to n-th travel paths P7 to Pn in order from the remaining portion of the first digging wall W1 to the remaining portion of the third digging wall W3.

FIG. 10 is an enlarged view of the first travel path P1 and the second travel path P2. As illustrated in FIG. 10 , the first travel path P1 includes a first digging path PA1, a first soil transportation path PB1, and a first reverse path PC1. The first digging path PA1 includes a straight line crossing the first digging wall W1 from the first slot S1 toward the second slot S2. The first digging path PA1 is inclined with respect to the predetermined work direction A1. The first digging path PA1 extends from a start position ST1 at the first slot S1 side to a position on the second slot S2.

The first soil transportation path PB1 is a straight line extending from the first digging path PA1 to the second dumping position D2. The first soil transportation path PB1 extends in the predetermined work direction A1 on the second slot S2. The controller 31 determines the first soil transportation path PB1 from the first digging path PA1 and the second dumping position D2. The first reverse path PC1 extends from the second dumping position D2 to a next start position ST2. The controller 31 determines the first reverse path PC1 from the first digging path PA1, the second dumping position D2, and the next start position ST2.

The second travel path P2 includes a second digging path PA2, a second soil transportation path PB2, and a second reverse path PC2. The second digging path PA2 includes a straight line crossing the second digging wall W2 from the second slot S2 toward the third slot S3. The second digging path PA2 is inclined with respect to the predetermined work direction A1. The second digging path PA2 extends from the start position ST2 at the second slot S2 side to a position on the third slot S3.

The second soil transportation path PB2 is a straight line extending from the second digging path PA2 to a third dumping position D3. The second soil transportation path PB2 extends in the predetermined work direction A1 on the third slot S3. The controller 31 determines the second soil transportation path PB2 from the second digging path PA2 and the third dumping position D3. The second reverse path PC2 extends from the third dumping position D3 to a next start position ST3. The controller 31 determines the second reverse path PC2 from the second digging path PA2, the third dumping position D3, and the next start position ST3. Similarly to the first travel path P1 and the second travel path P2, other travel paths also include a digging path, a soil transportation path, and a reverse path.

The controller 31 determines the start positions ST1 to STn based on, for example, the positions of the digging walls W1 to W3. The controller 31 may determine, as the start positions ST1 to STn, positions spaced apart by a predetermined distance from the digging wall W1 to W3. Alternatively, the controller 31 may set arbitrary start lines and determine positions on the start lines as the start positions ST1 to STn.

In step S105, the controller 31 controls the work machine 1 according to the travel paths P1 to Pn. For example, as illustrated in FIG. 10 , the controller 31 controls the work machine 1 according to the first travel path P1. Specifically, the controller 31 causes the work machine 1 to move along the first digging path PA1. The controller 31 causes the work machine 1 to move along the first soil transportation path PB1 subsequent to the first digging path PA1. The controller 31 causes the work machine 1 to move along the first reverse path PC1 subsequent to the first soil transportation path PB1. As a result, the work machine 1 digs a part or entirety of the first digging wall W1 and dumps the dug soil at the second dumping position D2.

The controller 31 controls the work machine 1 according to the second travel path P2 subsequent to the first travel path P1. Specifically, the controller 31 causes the work machine 1 to move from the start position ST2 along the second digging path PA2. The controller 31 causes the work machine 1 to move along the second soil transportation path PB2 subsequent to the second digging path PA2. The controller 31 causes the work machine 1 to move along the second reverse path PC2 subsequent to the second soil transportation path PB2. As a result, the work machine 1 digs a part or entire of the second digging wall W2 and dumps the dug soil at the third digging position D3.

Hereinafter, as illustrated in FIGS. 7 to 9 , the controller 31 controls the work machine 1 so that the work machine 1 travels along the first to n-th travel paths P1 to Pn in order and the digging walls W1 to W3 are dug with the work implement 13. As illustrated in FIG. 9 , the travel paths P7 to Pn when the last portions of each of the digging walls W1 to W3 are dug may extend in the predetermined work direction A1 along the digging walls W1 to W3, respectively. Alternatively, the travel paths P7 to Pn may extend in a direction inclined with respect to the predetermined work direction A1, similarly to other travel paths.

Next, processes for determining the digging path will be described below. FIG. 11 is an enlarged view of the m-th travel path Pm (1≤m≤n). The m-th travel path Pm is an arbitrary travel path of the first to n-th travel paths P1 to Pn. As illustrated in FIG. 11 , the m-th travel path Pm includes an m-th digging path PAm, an m-th soil transportation path PBm, and an m-th reverse path PCm.

FIG. 12 is a flowchart illustrating processes for determining the digging path. As illustrated in FIG. 12 , in step S201, the controller 31 determines candidate paths Mi,j of the m-th digging path PAm. The controller 31 determines the candidate paths Mi,j of the m-th digging path PAm based on the actual topography data. FIG. 13 is a view illustrating an example of the candidate paths Mi,j of the m-th digging path PAm. As illustrated in FIG. 13 , the candidate paths Mi,j of the m-th digging path PAm extend from each of reference points Ri (i=1, 2, . . . ) in a plurality of directions and are defined by straight lines crossing an m-th digging wall Wm.

The controller 31 determines a reference point Ri on the m+1-th slot Sm+1. The controller 31 determines one or more reference points Ri along the centerline of the m+1-th slot Sm+1. For example, the controller 31 determines positions at certain intervals along the centerline of the m+1-th slot Sm+1 as the reference points Ri. The candidate paths Mi,j of the m-th digging path PAm extend in different directions by a predetermined angle α. For example, the candidate paths Mi,j of the m-th digging path PAm may extend from a predetermined reference line in different directions by the predetermined angle α. The reference line may be determined in consideration of work efficiency. The predetermined angle α may be determined in consideration of the work efficiency and the calculation costs of the controller 31.

In step S202, the controller 31 calculates an evaluation function for each of the candidate paths Mi,j of the first digging path PAL The evaluation function is, for example, a function of the A* algorithm. The evaluation function may be a function of another path search algorithm such as the Dijkstra algorithm or the greedy algorithm. The evaluation function is represented by the following equation (1).

f(m)=g(m)+h(m)   (1)

The function g(m) represents work time of the work machine 1. h(m) represents the amount of remaining soil in the digging walls W1 to W3. The work time of the work machine 1 is, for example, the moving time from the start position STm to the next start position STm+1 via the m-th dumping position Dm+1. The controller 31 calculates the work time from, for example, the set vehicle speed, the moving distance, and the traction force of the work machine 1.

In step S203, the controller 31 determines a candidate path Mi,j having a smallest evaluation function f(m) of the plurality of candidate paths Mi,j as the m-th digging path. That is, the controller 31 determines the candidate path Mi,j having the smallest evaluation function f(m) of the plurality of candidate paths Mi,j as the m-th digging path PAm. The controller 31 determines the m-th travel path Pm based on the m-th digging path PAm.

In step S204, the controller 31 determines whether the remaining soil amount Vr of the digging walls W1 to W3 is equal to or less than a predetermined threshold Vth. The predetermined threshold Vth may be zero. The predetermined threshold Vth may be a small value to a degree that assumes that substantially all of the digging walls W1 to W3 has been dumped. When the remaining soil amount Vr of the digging walls W1 to W3 is not equal to or less than the predetermined threshold Vth, the process returns to step S201 and the controller 31 determines a next travel path. When the remaining soil amount Vr of the digging walls W1 to W3 is equal to or less than the predetermined threshold Vth, the controller 31 stops determining the travel path. That is, the controller 31 repeats determining the travel paths until the remaining soil amount Vr of the digging walls W1 to W3 is equal to or less than the predetermined threshold Vth.

In the system and method for controlling the work machine 1 according to the present embodiment described above, the plurality of candidate paths Mi,j crossing the digging walls W1 to W3 are determined. Then, a candidate path Mi,j having the smallest evaluation function f(m) of the plurality of candidate paths Mi,j is determined as the digging path PAm. Therefore, the digging work of the digging walls can be easily performed with the automatic control of the work machine 1.

Although one embodiment of the present invention has been described so far, the present invention is not limited to the above embodiment and various modifications can be made without departing from the gist of the invention. The work machine 1 is not limited to the bulldozer and may be another machine, such as a wheel loader. The travel device may include tires instead of the crawler belts. The work machine 1 may be a vehicle that can be remotely operated. In this case, the operating cabin may be omitted from the work machine 1.

A portion of the control system 3 may be disposed outside of the work machine 1. For example, the controller 31 may have a plurality of controllers 31 separate from each other. As illustrated in FIG. 14 , the controller 31 may include a remote controller 311 disposed outside of the work machine 1 and an onboard controller 312 mounted on the work machine 1. The remote controller 311 and the onboard controller 312 may be able to wirelessly communicate with each other via the communication devices 33 and 36. A portion of the above-mentioned functions of the controller 31 may be executed by the remote controller 311 and the remaining functions may be executed by the onboard controller 312. For example, the processes for determining the digging path may be executed by the remote controller 311 and the processes for causing the work machine 1 to move may be executed by the onboard controller 312.

The automatic control of the work machine 1 may be a semi-automatic control that is performed in combination with manual operation by an operator. Alternatively, the automatic control may be a fully automatic control that is performed without manual operation by an operator. For example, as illustrated in FIG. 14 , the work machine 1 may be remotely operated by an operator operating an operating device 37 disposed outside of the work machine 1.

The processes for performing digging work of the digging walls are not limited to the above-mentioned processes and may be changed. For example, some of the above processes may be changed or omitted. A process that is different from the above processes may be added to the processes for performing digging work of the digging walls.

The plurality of work machines 1 may simultaneously perform digging work of the digging walls. In this case, each of the controllers mounted on the plurality of work machines 1 may autonomously execute the above control. Alternatively, a controller that is common to the plurality of work machines 1 may execute the above control for the plurality of work machines 1.

The order of digging of the digging walls is not limited to that of the above embodiment and may be changed. For example, as illustrated in FIGS. 15 to 17 , the controller 31 may determine the travel paths P1 to P3 in order from a starting end to a terminating end of the first digging wall W1. Specifically, as illustrated in FIG. 15 , the controller 31 determines the first travel path P1 for the first digging wall W1. The first travel path P1 includes the first digging path PA1 crossing a first portion W11 of the first digging wall W1. The first portion W11 includes a starting end of the first digging wall W1.

As illustrated in FIG. 16 , the controller 31 determines the second travel path P2 for the first digging wall W1. The second travel path P2 includes the second digging path PA2. The second digging path PA2 is positioned in front of the first digging path PA1. The second digging path PA2 crosses a second portion W12 of the first digging wall W1. The second portion W12 is positioned in front of the first portion W11.

Subsequently, the controller 31 repeats determining the travel paths for the first digging wall W1 until the work machine 1 reaches the terminating end of the first digging wall W1. As illustrated in FIG. 17 , the controller 31 determines a k-th travel path Pk for the first digging wall W1. The k-th travel path Pk includes a k-th digging path PAk. The k-th digging path PAk crosses a k-th portion W1k of the first digging wall W1. The k-th portion W1k includes the terminating end of the first digging wall W1.

After the digging of the first digging wall W1 is finished, the controller 31 may determine the travel paths in order from a starting end to a terminating end of the second digging wall W2. Subsequently, the controller 31 may determine the travel paths in order from a starting end to a terminating end of the third digging wall W3. In the above embodiment, the number of digging walls (m) is three. However, the number of digging walls (m) may be less than three or greater than three.

The method for determining the candidate paths is not limited to that of the above embodiment and may be changed. For example, the reference point is not limited to being on the centerline of the m+1-th slot Sm+1 and may be positioned apart from the centerline. The evaluation function f (m) is not limited to the work time or the remaining soil amount and may include another parameter. For example, the evaluation function f(m) may include the moving distance or fuel consumption of the work machine 1. Alternatively, the evaluation function f(m) may include a tipping probability based on the gradient of the actual topography.

In the above embodiment, the smallest evaluation function is regarded as an optimal solution. However, a largest evaluation function may be regarded as an optimal solution. In this case, the controller may determine a candidate path having a largest evaluation function of the plurality of candidate paths as the digging path.

According to the present disclosure, the digging work of the digging walls can be easily performed with the automatic control of the work machine. 

1. A system for controlling a work machine, the system comprising: a position sensor configured to output position data indicative of a position of the work machine; and a controller configured to acquire the position data, the controller being configured to acquire actual topography data, the actual topography data including a position of a first slot extending in a predetermined work direction, a position of a second slot positioned adjacent to the first slot, and a position of a first digging wall positioned between the first slot and the second slot, determine a plurality of candidate paths crossing the first digging wall from the first slot toward the second slot and extending in respective directions, calculate an evaluation function of a path search algorithm for each of the plurality of candidate paths, determine a candidate path having an optimal evaluation function of the plurality of candidate paths as a first digging path, and control the work machine according to the first digging path.
 2. The system according to claim 1, wherein the controller is further configured to acquire a dumping position for the second slot, determine a first soil transportation path extending from the first digging path to the dumping position along the second slot, and control the work machine according to the first soil transportation path subsequent to the first digging path.
 3. The system according to claim 2, wherein the controller is further configured to determine one or more reference points on the second slot, and determine the plurality of candidate paths so as to extend from the one or more reference points in the respective directions.
 4. The system according to claim 2, wherein the actual topography data further includes a position of a third slot positioned adjacent to the second slot and a position of a second digging wall positioned between the second slot and the third slot, and the controller is further configured to determine a second digging path crossing the second digging wall from the second slot toward the third slot, and control the work machine according to the second digging path subsequent to the first soil transportation path.
 5. The system according to claim 2, wherein the controller is further configured to determine a second digging path positioned in front of the first digging path and crossing the first digging wall in the predetermined work direction, and control the work machine according to the second digging path subsequent to the first soil transportation path.
 6. A method for controlling a work machine, the method comprising: acquiring position data indicative of a position of the work machine; acquiring actual topography data including a position of a first slot extending in a predetermined work direction, a position of a second slot positioned adjacent to the first slot, and a position of a first digging wall positioned between the first slot and the second slot; determining a plurality of candidate paths crossing the first digging wall from the first slot toward the second slot and extending in respective directions; calculating an evaluation function of a path search algorithm for each of the plurality of candidate paths; determining a candidate path having an optimal evaluation function of the plurality of candidate paths as a first digging path; and controlling the work machine according to the first digging path.
 7. The method according to claim 6, further comprising: acquiring a dumping position for the second slot; determining a first soil transportation path extending from the first digging path to the dumping position along the second slot; and controlling the work machine according to the first soil transportation path subsequent to the first digging path.
 8. The method according to claim 7, further comprising: determining one or more reference points on the second slot; and determining the plurality of candidate paths so as to extend from the one or more reference points in the respective directions.
 9. The method according to claim 7, wherein the actual topography data further includes a position of a third slot positioned adjacent to the second slot and a position of a second digging wall positioned between the second slot and the third slot, the method further comprising: determining a second digging path crossing the second digging wall from the second slot toward the third slot; and controlling the work machine according to the second digging path subsequent to the first soil transportation path.
 10. The method according to claim 7, further comprising: determining a second digging path positioned in front of the first digging path and crossing the first digging wall in the predetermined work direction; and controlling the work machine according to the second digging path subsequent to the first soil transportation path. 